Certain instruments and products require a precise source of very high-voltage radio-frequency sine waves. One example is a mass spectrometer, in which the high-voltage RF source drives a substantially capacitive load consisting of a multi-polar (or “multipole”) electrode assembly such as a multi-polar rod assembly. One particular example is a quadrupole (or “quad”), which is formed by two pairs of rods each receiving a RF signal to generate a RF electric field in an interior space within the quad. The RF electric field focuses ions into an ion beam at the central axis of the space. The ions enter the space at one end of the space and travel along the central axis exiting at the opposite end of the space. In some implementations, a DC component is added to the RF signal at each rod pair to provide an ion mass bandpass filter.
The high voltage is typically generated using an oscillator to drive a radio-frequency power amplifier, which in turn drives a high-Q LC resonator tuned to the same frequency as the oscillator. The resonator magnifies the drive voltage from the power amplifier to produce the necessary high voltage. For a given instrument running a particular experiment, the desired frequency is normally fixed. The amplitude however must be settable over a wide range, such as for example 50V to 8 kVpp (peak-to-peak amplitude).
The use of a resonator driven by the RF amplifier advantageously allows the capacitance of the load to effectively operate as the majority of the circuit capacitance resonating with the inductance of the resonator. This substantially reduces the required power from the amplifier (typically by one to two orders of magnitude).
One problem with using the resonator with the RF amplifier is that the resonator frequency tends to drift. That is, the frequency to which the resonator is tuned changes over time. The result of this drift is that the resonator frequency no longer matches the oscillator, or drive frequency. When the resonator frequency fails to match the drive frequency, the voltage magnification factor provided by the resonator decreases. The magnification factor decrease results in a decrease in the high-voltage amplitude, which is problematic for spectrometer operation.
The amplitude may be maintained constant using a feed-back based circuit called a leveling loop. This common technique understood by those skilled in the art increases the amplitude of the oscillator signal being fed to the power amplifier input. This increases the amplifier output power to compensate for the reduced magnification factor from the out-of-tune resonator. The leveling loop is effective at keeping the high-voltage amplitude constant as long as the resonator frequency has not drifted more than about 0.5%, which corresponds to a capacitance or inductance change of about 1%. It would be desirable to cope with capacitor or inductor resonance changes over a range of greater than 1% without resorting to unreasonably large and expensive amplifiers that may raise other related issues.
The frequency drift in resonator-based RF drivers is mostly temperature related. The resonator physically heats up or cools down. The temperature changes may be the result of instrument components heating up and cooling down, the resonator itself dissipating power, and ambient air temperature changes. The resonator expands and contracts, which causes the inductance to increase and decrease respectively. The resonant frequency changes inversely with the square root of inductance. Thermally induced capacitance changes also occur.
Even with great care, it is not practicable to completely eliminate inductance and capacitance change over time. In the present state of the art the inductance change is minimized to the extent that is reasonable cost-wise, and then the effects of the remaining resonator frequency drift are dealt with via a leveling loop and a moderately over-sized power amplifier.
One solution involves mounting a variable speed fan under control of a field-programmable gate array in the housing of the mass spectrometer near the resonator to reduce temperature changes. Adding a fan however requires that additional space be provided. It may also be necessary to address added electrical and audible noise when the fan is switched on during operation. Also, a fan cannot eliminate temperature changes; it can only reduce the range of change.
Therefore, it would be desirable to minimize or even eliminate the frequency drift without requiring larger and more costly amplifiers, or electromechanical components that could introduce noise into the system.